In recent years, the size and image quality of industrial-use liquid crystal display devices are increasing. The market of digital signage is expanding, the digital signage that displays useful information in diverse public facilities such as displaying services and seat availabilities at stations, airports, etc. or displaying information in shopping arcades, schools, and companies, and demands for large-sized liquid crystal display devices are expanding.
In the background art as described above, for semi-outdoor instillation conditions in station platforms or the like in particular, damage to the surfaces of liquid crystal panels can be considered as likely to occur due to penetration of the liquid crystal panels by dust and dirt that exist in the outside air, water droplets such as rain coming into contact with the liquid crystal panels or due to objects such as empty cans, thrown rocks, etc coming into contact with the liquid crystal panels. Because of this, it is desired to provide more uniform image quality, higher reliability, and longer lifetime for liquid crystal display devices. Further, there are increasing demands for reductions in the volume, profile, weight, and costs of liquid crystal display devices adapted for larger screens.
For these demands, a method is considered in which the surface of a liquid crystal panel is completely and tightly closed using a transparent non-reflective plate such as an acrylic sheet (panel surface protection method).
Furthermore, when the temperature of a liquid crystal panel possibly exceeds the guaranteed operating temperature, a method is also considered in which the temperature is controlled so as to reduce the calorific value by lowering the luminance of fluorescent lamps.
However, when the surface of the liquid crystal panel is completely and tightly closed, it is necessary to provide a clearance (in the following description, an air layer) between the surface of the liquid crystal panel and the transparent non-reflective plate. Because of this, the heat generated from fluorescent lamps in a light source unit, inverter circuit boards, and a control circuit board is transferred to this air layer. At this time, a build up of heat occurs in the air layer because the thermal conductivity of the air layer (the coefficient of thermal conductivity is 0.026 W/mk) is not excellent and the thermal conductivity of the transparent non-reflective plate covering the surface of the liquid crystal panel is not excellent as well. Consequently, temperature rises or temperature variations occur in the liquid crystal panel, which hamper the provision of uniform image quality, high reliability, and longer lifetime.
In addition, because image quality is also reduced in the configuration that lower the luminance, a problem of market appeal arises in accurately displaying and conveying urgent traffic guides, for example.
Accordingly, it is not said that the above-mentioned methods are not adequate for liquid crystal display devices having dust-proof properties, which are installed in semi-outdoor public facilities such as stations.
Then, for example, JP3975506B2 (in the following, abbreviated as Patent Document 1) discloses a liquid crystal display device and a cooling method for the lamp unit of a liquid crystal display device. This technique relates to a device having a cooling mechanism in a backlight accommodating unit. In this device, as shown in FIG. 1(A), accommodating unit 102 that accommodates lamps 101 of the liquid crystal display device has dust-proof properties, in which the air heated by lamps 111 is introduced into cooling mechanism 103 to cool the air and the air is then returned to lamp accommodating unit 102. Consequently, this configuration exerts a certain effect as a configuration that cools tightly closed lamp accommodating unit 102 and that prevents the penetration of outside dust and dirt.
However, according to the cooling technique described in the above-mentioned Patent Document 1, the cooling mechanism itself is disposed inside the lamp accommodating unit in order to spread the heat generated from the lamps. This increases the volume and weight of the lamp accommodating unit. Further, because the fabrication method thereof becomes complicated, costs are increased. Accordingly, this configuration is a disadvantageous for meeting the recent demands for liquid crystal display devices.
Furthermore, as shown in FIG. 1(B), the embodiment for cooling the lamp unit of the liquid crystal display device disclosed in Patent Document 1 is restricted to a scheme in which air blower means 104 and U-shaped ducts 105 that are connected on the outside of lamp accommodating unit 102 are used to cool only the heat generated from fluorescent lamps 101.
In fact, as regards the ration for the quantity of heat that occurs throughout the liquid crystal display device, the quantity of heat from the light source unit is about 60%, and the quantity of heat generated from electronic circuit components such as a power supply unit, personal computer functioning unit, and control unit is about 40%.
Thus, there is a problem in which the cooling means alone for the fluorescent lamp unit disclosed in the Patent Document 1 is not sufficient to meet the cooling capability and high reliability requirements for satisfying the temperature specifications of all the electronic components that have been mounted, from the viewpoint of the overall liquid crystal display device. Further, for improving this cooling capability, there is also a problem in that a tight closing property, which is enough to prevent the penetration of dust and dirt flying in the outside air or water droplets, has to be maintained.